Patent publication WO 93/07022 discloses a driving system of this kind, of the series hybrid type, wherein the generator set comprises a thermal engine, a brushless poly-phased electric generator and an electronic converter which supplies the direct supply current under the monitoring of a voltage regulator. The driving set comprises one or more driving sets each comprising an electronic traction converter and a brushless poly-phased electric motor. An electronic control unit globally adjusts the operation of the system in response to an accelerator pedal, to signals provided by various sensors and with reference to a memory containing a representation of a one or more characteristic fields of the thermal engine. This unit controls the power of the thermal engine via action on the throttle valve, the resistant torque of the generator via action on the supply converter, and the power of the electric motor via action on the traction converter as a function of the position of the accelerator.
An inherent disadvantage of this system lies in the great complexity of the adjusting means, which have to use a high number of sensors of different types, process their signals in conjunction with several desired values and elaborate different adjustment variables which must be coordinated with each other. Such co-ordination is particularly complex in transient states, which are preponderant in motor vehicles. It is even more complex when the generator set comprises an accumulator battery intended to provide a balance of supply current.
Furthermore, the known systems generally necessitate the use of current sensors which are not very precise, which introduces margins of error into the adjustment and often requires additional adjustments in order to assure stability. There exists therefore a need for a simpler control method, capable of operating with a reduced number of sensors and avoiding as far as possible the use of current sensors.
A relatively simple system, disclosed in publication WO 94/27837, provides two distinct control circuits for the generator set and the driving set, each using a speed signal of the engine or motor of the corresponding set and the set-point signal provided by the accelerator. The coordination of the operating states of the two sets is achieved by a power adjusting circuit placed between the two sets. On the basis of a comparison between a power signal from the thermal engine and the set-point signal from the accelerator, this circuit switches onto the direct current supply circuit either a parallel resistor if it is a question of accelerating, or a series resistor if it is a question of slowing down. The effect of these actions is to modify the resistant torque of the generator, while the thermal engine still rotates at "full throttle". The generator set, which is thus enslaved by the driving set, reacts relatively slowly and this can affect the vehicle performance. Moreover, the electric resistors induce a certain dissipation of energy. Further, the speed of the thermal engine being always high has disadvantages from the point of view of noise, fuel consumption and driving comfort.